Diagram of initial board drawing:
Meghan's amended drawing (with a few less background images, and my best attempt at explaining how the system works from current knowledge):
The reference and return photodiodes will convert the light signals they receive into current; from that current, we need information about what wavelengths of light were received. From searching basic information about photodiodes, they have a response parameter (speed/time) determined by the materials out of which they are made, which affects sensitivity in different wavelength regions. For example, PIN photodiodes are faster than P-N photodiodes.
This example PIN photodiode seems to be sensitive to IR and visible light, and has a parameter
$ \frac{1}{2}$